1. Field of the Invention
The present invention relates to a permanent magnet rotating machine including a rotor having magnet insert holes for receiving permanent magnets, and more particularly, to a technique for reducing influence caused when a rotary shaft is inserted into a rotary shaft insert hole of the rotor.
2. Description of the Related Art
Generally, a permanent magnet motor having a rotor in which permanent magnets are inserted into magnet insert holes is used as a motor for driving a compressor which is installed, for example, in an air conditioner or a refrigerator. Such a permanent magnet motor is typically referred to as an “interior permanent magnet motor (IPM motor)”.
FIG. 11 shows a rotor 850 of a permanent magnet motor disclosed in Japanese laid-open patent publication No. 7-236239, which is shown in cross section (taken in a direction perpendicular to the axial direction).
The rotor 850 is formed of a plurality of laminated electrical steel. The periphery surface of the rotor 850 is formed of salient-pole portions 850A1 to 850A4 forming magnetic poles and recesses 850B1 to 850B4. A rotary shaft insert hole 859, magnet insert holes 851a to 851d, semi-tubular rivet insert holes 855a to 855d, interlocks 857ab to 857da and passage holes 858ab to 858da are arranged in the rotor 850.
A rotary shaft 860 having an outside diameter larger than the bore diameter of the rotary shaft insert hole 859 is shrink fitted or press fitted into the rotary shaft insert hole 859. The shrink fitting is effected by enlarging the bore diameter of the rotary shaft insert hole 859 by heating the rotor 850 and then inserting the rotary shaft 860 into the rotary shaft insert hole 859. The press fitting is effected by inserting the rotary shaft 860 into the rotary shaft insert hole 859 by pushing in the rotary shaft 860 with a strong force.
Permanent magnets 852a to 852d are press fitted into the magnet insert holes 851a to 851d. Semi-tubular rivets 856a to 856d for integrating the laminated electrical steel sheets are inserted into the semi-tubular rivet insert holes 855a to 855d. The interlocks 857ab to 857da elongated in the circumferential direction of the rotor serve to lock the electrical steel sheets when laminated. The passage holes 858ab to 858da are used as oil passages.
In the rotor 850 shown in FIG. 11, the rotary shaft 860 is inserted into the rotary shaft insert hole 859 by shrink fitting or press fitting. Therefore, when the rotary shaft 860 is inserted into the rotary shaft insert hole 859, the inner wall surface of the rotary shaft insert hole 859 is pressed by the outer surface of the rotary shaft 860, so that the outside diameter of the rotor 850 expands as shown in FIG. 11 by a broken line.
When the outside diameter of the rotor 850 expands, harmonic components of the induced electromotive force of a stator winding increase. As a result, iron loss caused by the harmonic components increases and the motor performance is deteriorated. Further, if the gap between the outer circumferential surface of the rotor 850 and the inner circumferential surface of the stator is unevenly narrowed, noise and vibration may increase.
Further, in the rotor 850 shown in FIG. 11, the permanent magnets 852a to 852d are press fitted into the magnet insert holes 851a to 851d. Therefore, when the rotary shaft 860 is inserted into the rotary shaft insert hole 859, stress acts upon the rotor in such a manner as to expand its outside diameter and is applied to the permanent magnets 852a to 852d via the magnet insert holes 851a to 851d. As a result, the permanent magnets 852a to 852d may be cracked or chipped. Particularly, when the rotary shaft 860 is shrink fitted into the rotary shaft insert hole 859, stress is produced by the difference between the thermal expansion coefficients of the rotor 850 and the permanent magnets 852a to 852d and also applied to the permanent magnets 852a to 852d. Therefore, there is an increased possibility that the permanent magnets 852a to 852d may be cracked or chipped.